Micromechanical structure with textured surface and method for making same

ABSTRACT

A method for forming sub-micron sized bumps on the bottom surface of a suspended microstructure or the top surface of the underlying layer in order to reduce contact area and sticking between the two layers without the need for sub-micron standard photolithography capabilities and the thus-formed microstructure. The process involves the deposition of latex spheres on the sacrificial layer which will later temporarily support the microstructure, shrinking the spheres, depositing aluminum over the spheres, dissolving the spheres to leave openings in the metal layer, etching the sacrificial layer through the openings, removing the remaining metal and depositing the microstructure material over the now textured top surface of the sacrificial layer.

This application is a division of application Ser. No. 08/294,389 filedAug. 23, 1994, now U.S. Pat. No. 5,510,156.

FIELD OF THE INVENTION

The invention pertains to suspended microstructures with texturedsurfaces to reduce sticking when they come in contact with otherstructures or surfaces and a method for fabricating such structures.

BACKGROUND OF THE INVENTION

Micromechanical structures for sensing a physical quantity such asacceleration, vibration or electrostatic potential are useful in manyapplications, including air bag deployment and active suspension inautomobiles, and guidance systems in military weapons, among others.

One method of fabricating suspended microstructures is generally termedbulk-micromachining. In bulk-micromachining, a block of material, suchas silicon, for example, is subtractively etched to remove material,leaving behind the desired microstructure shape suspended from theremainder of the substrate by very thin resilient connectors.Accordingly, in bulk-micromachining, the microstructure, the supportingportion of the substrate, and the thin connectors are monolithicallyconstructed of the same material. U.S. Pat. No. 4,711,128 (Boura)discloses one such bulk-micromachined suspended microstructure.

Another method of fabricating chips with suspended micromachinedmicrostructures is generally termed surface-micromachining.Surface-micromachining involves additive forming of the microstructureover a substrate. For instance, a sacrificial oxide spacer layer such assilicon dioxide is deposited over the surface of a substrate of a wafer.The sacrificial spacer layer is selectively etched to open up holes inthe spacer layer, down to the substrate, in which anchors for supportingthe microstructure will be formed. A thin film microstructure material,such as polysilicon, is deposited over the sacrificial layer. Themicrostructure material fills in the holes where the sacrificial layerhad been etched down to the substrate and contacts the substrate to formanchors for supporting the microstructure. Enough microstructurematerial is deposited to fill in completely the holes as well as to forma uniform layer over the top of the sacrificial layer. Themicrostructure material is then patterned into a desired shape byphotolithography. Finally, the sacrificial layer is removed (i.e.,sacrificed) by, for instance, wet etching, thus leaving behind amicrostructure suspended above the substrate by the anchors.International patent application publication No. WO93/25915, entitledMONOLITHIC CHIP CONTAINING INTEGRATED CIRCUITRY AND SUSPENDEDMICROSTRUCTURE and assigned to the same assignee as the presentapplication, discloses in detail one such method for manufacturing asurface-micromachined suspended microstructure.

FIG. 2 is a top plan view of an exemplary suspended microstructure. Themicrostructure comprises a bridge 112 suspended above a substrate 114 byfour corner anchors 116. The bridge comprises a central beam 118 havinga plurality of fingers 120 extending transversely therefrom. A suspendedpolysilicon stationary finger 122 is positioned parallel and adjacent toeach finger 120 of the bridge 112. Stationary fingers 122 are alsosuspended on anchors and cantilevered over the substrate, but aresubstantially stationary because of their smaller mass and shorterlength of extension beyond the anchor. FIG. 1 is a side view of thebridge 112 in which the stationary fingers 122 have been removed fromthe view in order not to obfuscate the illustration. FIG. 1 helpsillustrate the anchors 116 and the elevation of the bridge 112 above thesubstrate 114. The polysilicon of the stationary fingers 122 and thebridge 112 is electrically conductive. The stationary fingers areconnected via conductors embedded (i.e., formed) in the substrate tocomprise two electrical nodes. In particular, the stationary fingerswhich are to the left of the corresponding moveable fingers form a firstnode which is charged to a first voltage and the stationary fingerswhich are to the right of the corresponding moveable fingers form asecond node which is charged to a second voltage. The beam 112,including the moveable fingers, is a third node that is charged to athird voltage between the first and second voltages.

The first and second sets of stationary fingers and the moveable fingersform two capacitors. The two sets of stationary fingers form the firstplate of first and second capacitors, respectively, and the moveablefingers form the second plate of both of the capacitors. When the chipis subjected to a force, the beam 112 moves relative to the stationaryfingers 122, thus altering the capacitance between each stationaryfinger 122 and its corresponding moveable finger 120. Circuitry measuresthe change in aggregate capacitance, which is directly indicative of theacceleration to which the bridge is subjected. Preferably, the circuitryforms a closed loop including the beam, to provide a feedback signalwhich re-centers the beam when it is offset from its equilibriumposition by acceleration. During both fabrication and normal use afterfabrication, it is possible for the suspended moveable portion of amicrostructure, such as beam 112, to be subject to a force which willcause a portion of the beam 112 (e.g., one of the moveable fingers) tocome in contact with another portion of the chip. For instance, amoveable finger may contact a stationary finger, if subjected to astrong lateral force, or the bottom surface of the beam 112 may come incontact with the substrate, if subjected to a strong vertical force.Contact also may occur due to electrostatic attraction or, duringfabrication, due to liquid surface tension during drying after a wetetch step. Such contact is undesirable since sticking between contactingsurfaces, particularly when one or both of The surfaces is polysilicon,is likely. Once a portion of the suspended microstructure becomes stuckto another portion of the unit, it is very difficult to separate thetwo. Accordingly, sticking typically results in failure of the sensor.

International patent application publication No. WO93/25915 discloses amethod for fabricating a surface-micromachined suspended microstructurein which bumps are formed on the bottom surface of the suspendedmicrostructure. The bumps are formed by placing small hollows in the topsurface of the spacer layer over which the microstructure material isdeposited during fabrication by means of standard photolithography. Whenthe microstructure material is deposited over the sacrificial layer, themicrostructure material will fill in the hollows, forming bumps ofmicrostructure material on the bottom surface of the microstructure.When the sacrificial layer is removed, the bumps remain on the bottomsurface of the microstructure and serve to minimize the area of contact,between the microstructure and the substrate. If and when the bottomsurface of the microstructure comes into contact with the substrate dueto electrostatic forces, vertical acceleration, or liquid surfacetension, only the bumps will contact, the surface. By thereby minimizingthe area of contact the likelihood of sticking is reduced. Also, byminimizing contact area, the magnitude of any electrostatic attractionis lessened, thus decreasing the likelihood of sticking even if contactoccurs.

Generally, it is desirable to make the bumps as small as possible inorder to minimize the contact area of the bumps and also to allow alarge number of bumps to be placed throughout the bottom surface of thesuspended microstructure without significantly affecting the mass orgeometry of the structure. The minimum bump size which can be reliablyachieved using standard photolithographic procedures, as discussed ininternational patent application publication WO93/25915, is limited bythe process in use at the foundry where the chip is fabricated.Presently a typical fabrication process line can achieve a minimum bumpsize of about 1 micron diameter.

Today, typical suspended microstructure geometries employ a minimumdimension of around a micron or larger. Accordingly, such chips arecommonly fabricated in foundries with minimum size capabilities of 1micron or larger. While there are foundries which can achieve higherresolution, e.g., minimum sizes of half a micron or possibly less,creating such a foundry is a significant expense and typically is notjustifiable solely for the purpose of producing smaller bump sizes, if aless fine process is adequate in all other respects for the chip beingproduced.

SUMMARY OF THE INVENTION

The present invention provides for fine resolution texturing of surfacesof a microchip and particularly a microchip having a suspendedmicrostructure in order to reduce sticking without the need to employfine resolution photolithography.

In fabricating microstructures according to the present invention, amono-layer of polymer spheres is electrostatically adhered to thesurface to be textured, if it is an upwardly facing surface, or to anupwardly facing surface beneath the surface to be textured, if adownwardly facing surface. For example, if it is desired to texture thebottom surface of a surface-micromachined suspended microstructure, thespheres may be deposited over an underlying sacrificial layer prior todeposition of the microstructure layer. The polymer sphere mono-layer isdeposited, for instance, by spraying the wafer (already bearing thesacrificial layer) with a liquid colloidal suspension of the polymerspheres in which the spheres are electrostatically charged or ionized.The spheres adhere to the wafer surface on a molecular level. The waferis then rotated in a horizontal plane about an axis normal to the topsurface of the sacrificial layer to order the spheres in a regularpattern. The speed of rotation should be fast enough to dislodge anyspheres other than the first layer of spheres which are adhered directlyto the wafer so as to form a mono-layer of contacting spheres on thesurface. However, the speed should be slow enough not to createirregular voids between spheres in the mono-layer.

The spheres preferably are then shrunk by, for example, exposure to anoxygen (or other) plasma or by ion milling so as to create an orderedregular pattern of spheres on the top surface of the sacrificial layer,with regular spaces between them. A metal is then deposited over thewafer. The polymer spheres are then removed by an appropriate etchantwhich will etch through the spheres but not the metal. This leaves ametal pattern comprising metal depositions in the spaces between wherethe shrunken spheres had been, with the sacrificial layer exposed wherethe spheres had previously been. The sacrificial layer is then etchedwith the metal layer acting as an etch mask. This etching process willcreate hollows in the top surface of the sacrificial layer where theshrunken spheres previously had been. The hollows will have the diameterof the shrunk polymer spheres.

When the microstructure layer is deposited over the sacrificial layer,it will fill in the hollows, creating convex bumps of the bottom surfaceof the microstructure layer. The microstructure layer is then etchedinto the desired shape and the sacrificial layer is then removed byetching, thus leaving the suspended microstructure with convex bumps onits underside.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side view of the bridge portion of an exemplarymicrostructure.

FIG. 2 is a top plan view of the microstructure of FIG. 1.

FIG. 3 is a cross-sectional side view of an exemplary wafer during afirst stage of the fabrication method of The present invention.

FIG. 4 is a cross-sectional side view of an exemplary wafer during asecond stage of the fabrication method of the present invention.

FIG. 5 is a cross-sectional side view of an exemplary wafer during athird stage of the fabrication method of the present invention.

FIG. 6 is a cross-sectional side view of an exemplary wafer during afourth stage of the fabrication method of the present invention.

FIG. 7 is a cross-sectional side view of an exemplary wafer during afifth stage of the fabrication method of the present invention.

FIG. 8 is a cross-sectional side view of an exemplary wafer during asixth stage of the fabrication method of the present invention.

FIG. 9 is a cross-sectional side view of an exemplary wafer during aseventh stage of the fabrication method of the present invention.

FIG. 10 is a cross-sectional side view of an exemplary wafer during astage of an alternate embodiment of the fabrication method of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described with respect to FIGS. 3-9 whichillustrate a chip during various stages of the fabrication process of asurface micromachined sensor structure in accordance with the presentinvention. The invention, however, can also be adapted to abulk-micromachining process,

In at least one way of fabricating a surface micromachinedmicrostructure, a sacrificial silicon oxide layer 12 is deposited on asilicon substrate 10. As will be discussed in greater detail herein, thesilicon oxide layer 12 later will be selectively etched to createopenings through layer 12 down to the top surface of substrate 10. Thematerial from which the suspended sensor structure is to be formed,e.g., polysilicon, will then be deposited on the wafer to completelyfill in the openings as well as form a blanket layer over the topsurface of the sacrificial layer 12. The polysilicon will then beselectively etched into a desired shape. Finally, the sacrificial layer12 will be etched in an etching process which will etch through andcompletely remove layer 12 without affecting the polysiliconmicrostructure. The portions of polysilicon which filled in the openingsin sacrificial layer 12 down to the top surface of substrate 10 will bethe anchors which support the structure on the substrate surface. Theremainder of the polysilicon microstructure will be suspended from theanchors above the substrate, the space formerly occupied by the layer 12now being emptied.

However, before any etching of layer 12 to form the anchor openings, amono-layer of polymer particles, preferably spheres, is deposited on thetop surface of sacrificial layer 12. There are numerous acceptablepolymers from which to choose the sphere material. In the preferredembodiment, the material is a latex, and particularly, polystyrene.Polydevinylbenzine and polytoluene also are acceptable options. The sizeof the spheres will depend on the desired size of the variations (orbumps) in the surface to be textured. Polymer manufacturers can producepolymer spheres within extremely precise tolerances in both shape andsize and in almost an infinite variety of sizes. Polystyrene spheres,for example, are widely available in sizes starting at sub-microndiameters. In the preferred embodiment, spheres are employed since theyare readily available. However, particles of a wide variety of shapescould be used. In fact, cubes or other particles with flat surfaceswould be preferable in terms of providing bumps likely to have minimalcontact area. Of course, such shapes, are more complex to form thanspheres and thus are not as readily available.

An ordered mono-layer coating of these spheres is deposited, in thepreferred embodiment, by spraying the wafer with a liquid solutioncontaining the spheres in colloidal suspension. The spheres preferablybear an electrostatic surface charge so that they repel each otherslightly. The pH of the solution can be adjusted to alter theelectrostatic forces between the spheres. In at least one preferredembodiment of the invention, the solution is a 10% by weight sulfatepolystyrene latex in water with a surface charge density of 5.6microcoulombs per square centimeter. Due to molecular adhesive forces,the spheres tend to adhere at the points where they contact the surface.

Because the electrostatic repulsive forces of the spheres are typicallyfairly weak, additional spheres will deposit over the first layer ofspheres adhered to the wafer surfaces. Thus, to form a mono-layer, thewafer is then spun in a horizontal plane about an axis normal to therelevant surface to be coated, e.g., the top surface of the sacrificiallayer. The spinning action causes the spheres which are not in the firstlayer and, thus, which are not molecularly adhered to the wafer, to bethrown off, leaving a mono-layer of spheres on the surface. Theparticles on the substrate also order themselves into a regular patterndue to inter-particle interactions. The speed of the spinning should besufficiently fast to dislodge spheres overlying the first, adhered layerand to yield a densely packed ordered mono-layer of colloidal particles,but not so fast as to cause irregular voids to develop in the mono-layercoating. The goal is to form as nearly as possible a single layer ofspheres on the surface of sacrificial layer 12 in a regular pattern. Ithas been found that, for a 4-6 inch wafer, a spinning rate of 1200 rpmis satisfactory.

FIG. 3 illustrates the wafer at this point in the fabrication process.FIG. 3 shows the spheres 14 in contacting relationship. However, thespheres may not necessarily be contacting each other depending onvarious factors, such as the spin speed, the extant electrostaticforces, and the properties of the colloidal solution.

In an exemplary embodiment of a sensor structure in accordance with thepresent invention, the texturing comprises convex bumps on the bottomsurface of the microstructure that are approximately 0.2 to 0.3 micronsin diameter and are spaced apart from each other approximately 1 microncenter-to-center. However, the process described herein can be used tocreate bumps of essentially any desired size. Preferably, themicrostructure has a textured surface comprising bumps of less than 0.5microns in diameter. The bumps may have diameters smaller than 0.3microns and, more particularly, may have diameters smaller than 0.2microns.

The size of the spheres deposited on the surface of the layer 12 isdictated primarily by the desired center-to-center spacing of the bumps.As will become clear from the discussion below, if the spheres arecontacting each other at this point, the center to center spacing of thebumps formed by this process should be equal to the diameter of thespheres.

Once the mono-layer of tightly packed spheres 14 is adhered to the topsurface of layer 12, the wafer is exposed to an oxygen plasma which willoxidize the spheres and shrink them at a controlled rate. The spheresshould be shrunk to a size equal to the desired size of the bumps. Inthe illustrated embodiment of the invention, the bumps are 0.2-0.3microns. Accordingly, the spheres are shrunk to an approximate size of0.2-0.3 microns.

FIG. 4 illustrates a cross-section of the wafer after the oxygen plasmastep. As shown in FIG. 4, the exposure to oxygen plasma does notsubstantially affect the silicon dioxide layer 12 or the siliconsubstrate 10. Since the spheres have not moved but have only beenshrunk, the center-to-center spacing of the spheres is stillapproximately 1 micron, as it was before shrinkage. The spheres now,however, are no longer contacting each other since they have diametersof only approximately 0.2-0.3 microns.

The spheres alternatively may be shrunk by ion milling which also willnot substantially affect the sacrificial layer 12 or the siliconsubstrate 10 if the energy is kept within reasonable bounds.

The next step is to sputter coat a metal layer (aluminum, in theillustrated embodiment) on the top surface of the wafer in order tocover the top surface of layer 12 and the spheres 14. FIG. 5 illustratesa cross-section of the wafer after this step. The aluminum layer isillustrated at 16.

Next, the spheres are dissolved, for example, by immersing the wafer inan appropriate corrosive chemical bath; a toluene bath works well withpolystyrene spheres. The toluene bath will dissolve the polystyrenespheres without affecting the aluminum 16, the sacrificial layer 12, orthe substrate 10. Although the toluene bath will not affect thealuminum, per se, the portions of aluminum, which overcoat the spheres14 will be released since they are bonded to the spheres and to nothingelse. Accordingly, after the toluene bath, the spheres will be dissolvedleaving in their places gaps 18 in the aluminum coating 16 oversacrificial layer 12, as illustrated in FIG. 6.

The patterned aluminum layer is now used as a mask for plasma etchingthe sacrificial layer 12. The plasma etching process will etch throughthe exposed portions of the silicon dioxide sacrificial layer 12 muchfaster than through the aluminum layer 16. The duration of the plasmaetching process is selected so as to etch a distance into the exposedregions of the sacrificial layer 12 equal to the desired depth of thebumps on the bottom surface of the microsensor structure. The thicknessof the aluminum layer which was previously deposited is selected to bethick enough as to not be etched completely through its thickness duringthe plasma etching of the sacrificial layer 12.

The aluminum layer is then removed by using standard aluminum wet or dryetching process. The etch process should be selected so as to etchthrough the aluminum without affecting the sacrificial layer or thesubstrate. Anchor openings which extend completely through thesacrificial layer down to the substrate are also etched through thesacrificial layer. This is done in a separate process with a separatemask and may be done at this point or any appropriate time earlier inthe fabrication process. FIG. 7 is a side cross-section of the wafer atthis point in the fabrication process. As shown, the sacrificial layer12 now has hollows 19 formed in its top surface.

At this point, the material of the microstructure, e.g., polysilicon, isdeposited over the sacrificial layer filling in the anchor opening andthe hollows 18 in the top surface of the sacrificial layer 12. Enoughpolysilicon is deposited to fill in completely the anchor openings andthe hollows, and to form a blanket above the top surface of thesacrificial layer of a thickness equal to the desired thickness of themicrostructure, e.g., 2 micron. The polysilicon may be deposited by alow pressure chemical vapor deposition (LPCVD) process or any otheracceptable process. FIG. 8 illustrates the cross-section of the wafer atthis point in the fabrication process. FIG. 8 shows an anchor opening 20which is filled by anchor 22, the main layer 24 of the polysiliconstructure and bumps 26 on the bottom surface of the main layer of thepolysilicon structure.

At this point, the polysilicon structure is still supported by thesacrificial layer 12. The polysilicon layer is now etched into thedesired final microstructure shape, such as illustrated in FIG. 2. Thisetching can be done by a dry etch process or any other acceptableprocess.

Once the microstructure is fully shaped, the sacrificial layer is etchedcompletely away, leaving the microstructure suspended over the substrate10 by anchors 22. FIG. 9 illustrates a cross-section of the wafer afterthis final microstructure release step. As shown in that figure, thebottom surface of the main layer 24 of the microstructure includesregularly spaced bumps 26 which will minimize contact area with thesubstrate should the microstructure be bent down by mechanical,electrostatic or any other force to contact the substrate.

It should be understood by those skilled in the art that, while theabove discussion describes a preferred set of steps for creating atextured surface in accordance with the present invention, the actualprocess for fabricating a suspended microstructure on a wafer has beensimplified in this discussion so as not to obfuscate the invention. Inpractice, many additional steps will probably be needed which are notexpressly discussed herein but which are clearly understood by thoseskilled in the art. Additional steps will particularly be needed ifcircuitry also is to be monolithically incorporated on the wafer.

The present invention is intended to be incorporated generally into theoverall monolithic accelerometer fabrication process described ininternational patent application publication No. WO93/25915.Accordingly, it should be understood that a significant number of stepswhich are not discussed herein occur prior to the deposition of thespheres. The most notable of those steps, of course, would be thedeposition of the sacrificial layer 12 on the substrate 10. Further,steps relating to other aspects of the fabrication of a chip containinga suspended microstructure may be dispersed intermediate the stepsdescribed in this disclosure. In particular, it is likely that manysteps which are not discussed in this disclosure occur between theetching of the polysilicon into the desired shape and the final beamrelease by removal of the sacrificial layer, i.e., between the stagesillustrated by FIGS. 8 and 9.

For instance, it is envisioned that the steps of deposition of thespheres onto the top surface of sacrificial layer 12 up to the plasmaetching of the bumps would replace process 43 in the procedure disclosedin international patent application publication No. WO93/25915. Further,the process of constructing the microsensor would be performed generallyin accordance with processes 44-49 described in international patentapplication publication No. WO93/25915 and that steps generally inaccordance with processes 50-66 discussed in international patentapplication publication No. WO93/25915 will be performed after thedeposition of the polysilicon layer, but before the final release of thebeam. The disclosure of international patent application publication No.WO93/25915 which corresponds to U.S. Pat. No. 5,326,726 issued Jul. 5,1994, is incorporated herein by reference

As an alternative to the above-described preferred process, upwardlyfacing bumps 51 can be formed on the top surface of silicon layer 10rather than, or in addition to, forming bumps on the bottom surface ofthe polysilicon main layer 24. This would be accomplished by depositingthe spheres on the silicon layer prior to deposition of the sacrificiallayer 12. The spheres are shrunk as previously described. At this point,a dry etching process should be selected which etches through both thespheres and the silicon layer, which etching is stopped when the spheresare completely dissolved. This will leave a silicon surface havingupwardly extending bumps, as illustrated in FIG. 10. The sacrificialoxide layer can then be deposited and the fabrication procedurecontinued as generally described in international patent applicationpublication No. WO93/25915.

Having thus described a few particular embodiments of the invention,various alterations, modifications and improvements will readily occurto those skilled in the art. Such alterations, modifications andimprovements as are made obvious by this disclosure are intended to bepart of this description though not expressly stated herein, and areintended to be within the spirit and scope of the invention.Accordingly, the foregoing description is by way of example only, andnot limiting. The invention is limited only as defined in the followingclaims and equivalents thereto.

I claim:
 1. A micromachined device comprising a microstructure suspendedabove a substrate, said microstructure having a textured surfacecomprising bumps having diameters less than 0.5 microns, withcenter-to-center spacings between said bumps greater than the diametersof said bumps.
 2. A micromachined device as set forth in claim 1 whereinsaid bumps have diameters smaller than 0.3 microns.
 3. A micromachineddevice as set forth in claim 1 wherein said bumps have diameters smallerthan 0.2 microns.
 4. A micromachined device as set forth in claim 2wherein said microstructure is formed of polysilicon and said substrateis formed of silicon.
 5. A micromachined device comprising amicrostructure suspended above a substrate, said microstructure having atextured surface comprising bumps having diameters less than 0.5microns, with center-to-center spacings between said bumps greater thanthe diameters of said bumps, said device fabricated in accordance with amethod comprising the steps of:(1) depositing a sacrificial layer on asubstrate, said layer having a top surface, (2) depositing a mono-layerof particles of a first size on said surface of said sacrificial layer,(3) shrinking said particles to a second size smaller than said firstsize, (4) depositing a layer of mask material over said sacrificiallayer and said particles, (5) removing said particles such that portionsof said mask material overlying said particles are removed whileportions not overlying said particles remain on said surface, (6)etching said sacrificial layer using said mask material as an etch maskso as to form hollows in said surface of said sacrificial layer, (7)removing said mask material, (8) removing portions of said sacrificiallayer to form holes in said sacrificial layer for formation of anchorsfor said microstructure; (9) depositing a microstructure material oversaid substrate and said sacrificial layer to form said microstructure,said microstructure material filling the holes in said sacrificial layerto form said anchors, and (10) removing said sacrificial layer, asurface of said microstructure material that was in contact with the topsurface of said sacrificial layer before removal of said sacrificiallayer having said bumps corresponding to said hollows, wherein adimension of said bumps is defined by the second size of said particlesand a spacing between said bumps is defined by the first size of saidparticles.
 6. A micromachined device comprising a microstructuresuspended above a substrate, said microstructure having a texturedsurface comprising bumps having diameters less than 0.5 microns, withcenter-to-center spacings between said bumps greater than the diameterof said bumps, said device fabricated in accordance with a methodcomprising the steps of:depositing a sacrificial layer on a substrate,said layer having a top surface, spraying said substrate bearing saidsacrificial layer with a solution in which polystyrene spheres of afirst size are in suspension, spinning said substrate about an axisperpendicular to said top surface of said sacrificial layer while insaid solution, removing portions of said sacrificial layer so as tocreate holes in said sacrificial layer for formation of anchors for saidmicrostructure, exposing said spheres to oxygen plasma so as to shrinksaid spheres to a second size smaller than said first size, depositing alayer of aluminum over said sacrificial layer and said spheres,immersing said substrate bearing said spheres in a bath that iscorrosive to said spheres so as to remove said spheres and a portion ofsaid mask material overlying said spheres, plasma etching saidsacrificial layer using said aluminum as an etch mask so as to formhollows in said top surface of said sacrificial layer, removing saidaluminum by etching, depositing a microstructure material over saidsubstrate and said sacrificial layer to form said microstructure, saidmicrostructure material filling the holes in said sacrificial layer toform said anchors, and removing said sacrificial layer, a surface ofsaid microstructure material that was in contact with the top surface ofsaid sacrificial layer before removal of said sacrificial layer havingsaid bumps corresponding to said hollows, wherein a dimension of saidbumps is defined by the second size of said spheres and a spacingbetween said bumps is defined by the first size of said spheres.